Case hardening of steel



1967 A. G. HAYNES CASE HARDENING OF STEEL Filed Oct. 11, 1965 INVENTOR. 4440 QEOPGC' #470215 BY 7w 1 Aim/Y5) FME CASE

1 m u m Mmm United States Patent 3,345,220 CASE HARDENING OF STEEL Alan George Haynes, Sevenoaks, England, assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware Filed Oct. 11, 1965, Ser. No. 494,335 Claims priority, application Great Britain, Jan. 18, 1965, 41,077/65 4 Claims. (Cl. 148148) The present invention relates to the case hardening of steel and, more particularly, to a process for case hardening steel so as to produce optimum properties in both case and core.

It is well known that alloys possessing very high hardness tend to be brittle and susceptible to rapid fracture under load, and thus an article or part made entirely from such an alloy would not perform satisfactorily under impact or other types of rapidly applied stresses. This is particularly true in steels Where high carbon contents are employed in order to obtain high hardness. To overcome this inherent problem, the process of case hardening has been developed for steels.

In one process for case hardening steel, the surface of an article or parts made from a low-carbon steel is enriched in carbon by heating the article or part in contact with a carburizing medium. During this carburizing treatment, carbon is caused to diffuse into the low-carbon steel to produce an enriched layer, usually between 0.005 inch and 0.100 inch deep depending upon the ultimate use for the article or part. The carbon-enriched layer, known as the case, usually contains about 0.6% to about 1.3% carbon, while the low-carbon portion remaining, known as the core, usually contains only about 0.05% to about 0.27% carbon.

The actual case hardening is effected by cooling the carburized article or part from a temperature at which the carburized case is completely or substantially austenitic (austenitizing temperature) to a temperature at which the case transforms to martensite. The attainment of this austenitizing or hardening temperature may be efiected in several ways. The article or part can be carburized at the austenitizing temperature or at a higher temperature and cooled directly to cause transformation to martensite, or the article or part can be reheated to an austenitizing temperature from room temperature after previous carburizing or refining heat treatments.

The aim in cooling from the austenitizing temperature is to produce a hard martensitic microstructure in the carbon-enriched zone at the surface. Most case-hardening steels need to be quenched in oil, water or molten salts at lowtemperatures in order to minimize or suppress the transformation of austenite in the case to undesirably soft microstiuctures which occur at intermediate temperatures. The resulting martensitic case is very hard, having a hardness above Rockwell C 60 and usually about Rockwell C 64.

On the other hand, the low-carbon core remains at a lower hardness level after quenching and thereby retains a high resistance to impact. The effect of case hardening is, therefore, to produce a very hard, wearresistant surface (case) which is also unavoidably brittle, and to combine this with a core which has good ductility although of lower hardness.

Although some relief of internal stress of the case can be effected by heating at a temperature below 200 C., e.g., in the range 120 C. to 180 C., the brittleness of the hard, high-carbon, martensitic case persists unless it is tempered by reheating to temperatures Well in excess of 200 C. Such tempering, however, even in the range of 200 C. to 250 C. has the undesirable effect of causing marked loss of hardness, and reducing any 3,345,220 Patented Oct. 3, 1967 "ice favorable compressive stresses. Thus, such tempering treatments are of very limited value.

The hardness and tensile strength of the low-carbon core are, on the other hand, far less affected by tempering, and low-carbon steels with good depth-hardening characteristics can be tempered at 350 C. or even 400 C. without impairment of these properties. Steels with somewhat lower depth-hardening characteristics can be tempered at temperatures as high as 500 C. before there is a loss of strength and hardness greater than approximately 10%. p

A problem arises with respect to case-hardened articles,

however, in that a stress which is applied to the surface of such an article is transmitted, at least in part, from the case to the core. The case, having a high hardness, generally has higher strength and will perform completely elastically even when the stress is of such magnitude as to cause plastic deformation of the core. The onset of plastic deformation will occur when the applied stress exceeds the measured elastic limit and the proportional limit of the core material. Any such plastic deformation which occurs in the core tends to prevent the case from returning to its original position or shape when the externally applied stress is released. Such a result tends to promote unfavorable stress patterns in the case, and these can ultimately lead to failure of the article or part in question.

' On the other hand, if the resistance of the core to plastic deformation can be increased under conditions which do not alter the hardness and strength of the case, both the core and the case will perform elastically under high stress, and articles or parts having such a combination of core and case can be subjected to higher stresses than the same parts given a conventional case-hardening treatment, Without fear of failure under load.

It has now been discovered that by transforming the austenite in the core of a carburized article or part and tempering the transformation product prior to transforming the case to martensite, there results an article or part having a case and core capable of performing elastically under high stress but which retains the hardness and wear resistance characteristic of the case of ,a conventionally case-hardened article or part.

' It is an object of the present invention to provide a process for case-hardening carburized articles so as to produce a case and core capable of performing elastically under high stress without sacrifice of hardness or Wear resistance in the case.

Another object of the invention is to provide a case-' hardened article of manufacture having case and core portions capable of performing elastically under high stress together with a hard and wearresistant case portion.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing which depicts, schematically, the case-hardening heat treatment of the present invention, superimposed on part of the isothermal transformation diagrams for the case and core portions of a typical carburized article or part as contemplated herein.

Generally speaking and in accordance with the present invention, an article or part on which a case has been formed by carburization is quenched to a temperature at which austenite transformation in the core is complete or substantially complete, but at which the case remains 'austenitic. The part is then heated from this quenching temperature to a higher temperature at which the case remains austenitic but at which the core is tempered; held at this higher temperature to temper the core; and then cooled to convert the case to martensite. The process of the present invention puts to advantageous use the fact that although the ultimate tensile strength of the core of a carburized part is not greatly affected by tempering at temperatures even as high as 400 C. to 500 C., the elastic limit and proportional limit of the core will be significantly increased by such a treatment. That is to say, the core will perform elastically at high levels of external stress as compared with an untempered core.

The manner, according to the present invention, in which the core portion of an article or part can be tempered without sacrifice in the hardness or abrasion resistance of the case can best be seen with reference to the accompanying drawing in which isothermal transformation diagrams for the core portion (dotted lines) and the case portion (solid lines) of typical carburized articles according to the present invention are set forth. These diagrams illustrate, schematically, the temperature at which martensitic transformation begins (Ms) and at which martensitic transformation is essentially complete (Mf) for both the case and core portions. It can be seen that the carbon enrichment of the case lowers the temperature range in which austenite transforms on cooling, so that the case transforms at appreciably lower temperatures than the core.

To begin the process of the present invention, the carburized article is quenched from an austenitizing temperature A to temperature B, which is above the Ms temperature of the case. If the alloy content and cooling conditions are such that martensite can be produced in the core, the start of transformation of the core is normally in the region of 40 C. and is complete or substantially complete when the part has been cooled to 200 C. or even only to 250 C. If the core has a lower alloy content and thus lower depth hardenability, then transformation of austenite to the softer bainitic or lowerpearlitic types of structure will be complete, or substantially complete, at temperatures of about 300 C. and may be complete at considerably higher temperatures. With most carburized steels, the first quenching temperature should advantageously be in the range 200 C. to 250 C. and this quenching may advantageously be effected in a molten salt bath, though a hot oil bath may also be used. Generally speaking, the period of immersion in the bath need only be long enough to allow equalization of temperature (point B to point C on the accompanying drawing), e.g., about 1 to about 5 minutes.

The article or part is then reheated directly from the initial quenching temperature to the tempering temperature of the core, without allowing the article or part to cool to a temperature at which transformation of the case would occur. In this second step, the temperature of tempering of the core should be far enough above the initial quenching temperature to bring about a significant increase in the elastic limit of the core, but should not exceed the Ac temperature of the core. Merely holding the article or ,part at the initial quenching temperature does not have any significant effect in this respect. Subject to this, the tempering temperature and the duration of the tempering cycle are determined mainly by the isothermal transformation characteristics of the carbon-enriched austenite of the case. The time and temperature should be so chosen as to avoid undesirable high-temperature transformation products in the case, and to insure that the case transforms to martensite when it is finally cooled. This tempering can be effected by immersion in another salt bath. Generally speaking, a tempering temperature in the range 300 C. to 500 C. will be found suitable and it is most preferred that the tempering temperature exceed the prior quenching temperature by at least 30 C. and advantageously by at least 60 C. Points CDE or CDD, on the accompanying drawing, illustrate, respectively, a low and an intermediate temperature tempering step performed on the core. Higher tempering temperatures in the range above 500 C. up to the Ac temperature would not normally be employed because they lead to overtempering and undue softening of the core.

The final step of the process of the present invention comprises cooling from the tempering temperature. This cooling may be effected by cooling in air or by quenching in oil. The desirable hard martensitic structure of the case does not begin to form until the austenite of the case has been cooled to a temperature in the region of C. to 250 C. Transformation of austenite to martensite is not Wholly completed at one temperature, and normally the article or part has to be cooled to a temperature that is between C. and 200 C. below the Ms temperature of the case in order to render the case substantially or wholly martensitic. With regard to the accompanying drawing, this final cooling step is illustrated by points D"EF or EF, respectively, depending upon whether the article or part is finally cooled from a high or low core-tempering temperature.

In carrying the invention into practice, it is preferred to use as materials, steels of medium or high alloy content that can be carburized to give a case containing about 0.6% to about 1.3% carbon and in which austenite is stable throughout a wide range of time and temperature as shown by the isothermal transformation diagram. Such steels may contain one or more of nickel up to about 5%, mg, 0.1% to 4.5% nickel, chromium up to about 2%, e.g., about 0.05% to 1.75% chromium, molybdenum up to about 0.5%, erg, about 0.05% to 0.4% molybdenum, and manganese up to about 2%, e.g., 0.25% to 1.5% manganese. The use of the upper limits for two or more of these elements is not recommended and where the upper limit is used, then the amounts of the other constituents, if any, should be kept quite low. However, as will be seen below, amounts of two or more such elements in the middle of their ranges can be present. For practical purposes, the minimum incubation period for transformation to bainite, at a carbon content typical of the case, should preferably not be less than 100 seconds at 350 C.

Exemplary steels suitable for use in the present invention include those of the AISI types 3300, 4300, 4 600, 4700, 4800 and 9300 and those of British Standards which follow:

En 36 consisting essentially of carbon 0.12% to 0.18% silicon 0.010% to 0.35%, manganese 0.30% to 0.60%, nickel 3.0% to 3.75%, chromium 0.60% to 1.1% and optionally molybdenum 0.10% to 0.25%, balance iron;

En 39A consisting essentially of carbon 0.12% to 0.18%, silicon 0.10% to 0.35%, manganese 0.5% max., nickel 3.8% to 4.5%, chromuim 1.0% to 1.4%, balance iron;

En 39B consisting essentially of carbon 0.12% to 0.18%, silicon 0.10% to 0.35%, manganese 0.5% max., nickel 3.8% to 4.5%, chromium 1.0% to 1.4%, molybdenum 0.15% to 0.35%, balance iron;

En 354 consisting essentially of carbon 0.2% max., silicon 0.35% max., manganese 0.50% to 1.0%, nickel 1.5% to 2.0%, chromium 0.75% to 1.25%, molybdenum 0.10% to 0.20%, balance iron; and

En 355 consisting essentially of carbon 0.2% max., silicon 0.35% max., manganese 0.40% to 0.70%, nickel 1.8% to 2.2%, chromium 1.4% to 1.7%, molybdenum 0.15% to 0.25%, balance iron.

For the purpose of giving those skilled in the art a better understanding of the invention and/or a better appreciation of the advantages of the invention, the following illustrative example is given:

Example A steel (BS En 39B) nominally containing about 4.25% nickel, about 1% chromium, about 0.2% molybdenum, about 0.17% carbon, balance essentially iron, has Ms and Mftemperatures of about 400 C. and 200 C., respectively. When carburized to form a case containing 0.9% carbon, the Ms temperature of the case will be lowered to a temperature in the approximate range 100 C. to 150 C. According to the invention, this steel, after carburizing, may be quenched in a bath in and held in this bath for from about 3 to about 10 minutes or even longer, e.g., up to about 30 minutes, without causing transformation of the case.

Specimens of the steel set forth hereinabove were gas curburized to produce a case about 0.03 to about 0.04 inch deep and having a carbon content of approximately 1%. Several of the specimens (-Nos. 1 to were oil quenched from the carburizing temperature of 925 C. to room temperature to transform both the core and the case to martensite. Samples Nos. 2 to 5 were then tempered by heating at various temperatures in the range 200 C. to 500 C. for 30 minutes and finally cooled to room temperature. The treatment given Specimen No. 1 is typical of a conventional case-hardening heat treatment for carburized steel.

Other specimens of the same steel (Nos. 6 and 7) were heat treated in accordance with the present invention by quenching from the carburizing temperature of 925 C. into a salt bath at 200 C., at which temperature transformation in the core was substantially complete but at which temperature no transformation occurred in the case. The specimens were then immediately reheated to 250 C. and 500 C., respectively, for 30 minutes and finally quenched to room temperature in an oil bath.

All the specimens (Nos. 1 to 7) were final-1y cooled in liquid nitrogen to decompose any residual austenite. After treatment, the hardness of the case and core of each specimen was determined and the results are set for the in Table I, each value of data being the mean of several determinations.

The data set forth in Table I for Specimens Nos. 2 to 5 show that tempering to soften the core after a conventional case-hardening treatment results in very marked softening of the case-hardened surface of an article or part. In contrast to this, however, heat treatment according to the present invention (Specimens Nos. 6 and 7) successfully tempers the core, as shown by the lower hardness value of the core, while the case retains a hardness substantially equal to that of a conventionally casehardened material (Specimen No. 1).

It will be appreciated that the heat treatment according to the invention may be followed by a further low temperature tempering at a temperature below 200 C., e.g., at 120 C. to 180 C., to stress relieve the case without appreciably softening it.

The present invention is particularly applicable to the production of gears and other articles and parts subjected in use to high surface loadings.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A process for case-hardening a carburized steel article so as to produce a case and core capable of performing elastically under high stress without sacrifice of hardness or wear resistance in the case, comprising quenching said article from a temperature within the austenitizing temperature range to a first temperature at which austenite transformation in the core is substantially complete but at which the case remains substantially austenitic, heating said article to a second temperature sulficiently high to temper said core but below the AC temperature of the core, holding at said second temperature for a period of time sufiicient only to temper said core while maintaining said case substantially austenitic and thereafter cooling said article to transform said case to martensite.

2. A process as set forth in claim 1 in which the case of the carburized article contains about 0.6% to about 1.3% carbon and the core contains about 0.05% to about 0.27% carbon.

3. A case-hardened article of manufacture produced by the process of claim 1 in which the case is substantially martensite and the core is substantially a tempered transformation product of austenite.

4. A process for increasing the resistance of the core of "a carburized steel to plastic deformation such that the core and case of the steel are capable of performing elastically under higher stresses without the incurrence of deleterious hardness or wear resistance in the case which comprises quenching said steel from a temperature within the 'austenitizing temperature range to a temperature within the range of about 400 C. to about 200 C. at which austenite transformation of the core is substantially complete but at which the case remains substantially autenitic, heating the steel to a second temperature sufiiciently high to temper the core, the temperature being at least 30 C. above the said quenching temperature but below the AC temperature of the core, holding at said second temperature for a period of time sulficient to temper the core only while maintaining said case substantially austenitic, and thereafter cooling said steel to transform said case to martensite.

References Cited UNITED STATES PATENTS 2,128,621 8/1938 Queneau l4839 X 2,207,298 7/ 1940 Fleischmann 148-165 2,585,372 2/1952 Day et a1. 148-16.5 X

FOREIGN PATENTS 1,191,401 4/1965 Germany.

762,948 12/ 1956 Great Britain.

OTHER REFERENCES Metals Handbook, 1948 edition, published by ASM pp. 681-685.

CHARLES N. LOVELL, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,345,220 October 3, 1967 Alan George Haynes It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 28, for "40 C." read 400 C.

column 4, line 42, for "silicon 0.010%" read silicon 0.10% line 47, for "chromuim" read chromium column 5, line 9, for "curburized" read carburlzed line 33, for "for the" read forth same column 5, TABLE I, in the heading to the sixth column, for "N read N column 6, line 43, for "autenitic" read austenltlc Signed and sealed this 11th day of February 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. A PROCESS FOR CASE-HARDENING A CARBURIZED STEEL ARTICLE SO AS TO PRODUCE A CASE AND CORE CAPABLE OF PERFORMING ELASTICALLY UNDER HIGH STRESS WITHOUT SACRIFICE OF HARDNESS OR WEAR RESISTANCE IN THE CASE, COMPRISING QUENCHING SAID ARTICLE FROM A TEMPERATURE WITHIN THE AUSTENITIZING TEMPERATURE RANGE TO A FIST TEMPERATURE AT WHICH AUSTENITE TRANSFORMATION IN THE CORE IS SUBSTANTIALLY COMPLETE BUT AT WHICH THE CASE REMAINS SUBSTANTIALLY AUSTENITIC, HEATING SAID ARTICLE TO A SECONDTEMPERATURE SUFFICIENTLY TO TEMPER SAID CORE BUT BELOW THE AC1 TEMPERATURE OF THE CORE, HOLDING AT SAID SECOND TEMPERATURE FOR A PERIOD OF TIME SUFFICIENT ONLY TO TEM- 